The present invention generally relates to a dual-hardness polishing pad for linear polishing and a method for fabrication and more particularly, relates to a dual-hardness polishing pad for use in a linear chemical mechanical polishing process consisting of a body portion formed of a hard material and a cover portion formed of a soft material, and a method for fabrication.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in memory devices. The planarization process is important since it enables the use of a high resolution lithographic process to fabricate the next level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile onto the wafer surface. The axis or rotation of the wafer and the axis of rotation of the pad are deliberately not collinear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 xc3x85 and about 10,000 xc3x85. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates and different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
More recently, a new chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefor named as a linear chemical mechanical polishing process in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a uniform polishing rate across a wafer surface throughout a planarization process for uniformly removing a film player of the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus and therefore not only reducing the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical linear CMP apparatus 10 is shown in FIGS. 1A and 1B. The linear CMP apparatus 10 is utilized for polishing a semiconductor wafer 24, i.e. a silicon wafer for removing a film layer of either an insulating material or a wafer from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.
As shown in FIGS. 1A and 1B, the wafer 24 is mounted on a rotating platform, or wafer holder 18 which rotates at a pre-determined speed. The major difference between the linear polisher 10 and a conventional CMP is that a continuous, or endless belt 12 is utilized instead of a rotating polishing pad. The belt 12 moves in a linear manner in respect to the rotational surface of the wafer 24. The linear belt 12 is mounted in a continuous manner over a pair of rollers 14 which are, in turn, driven by a motor means (not shown) at a pre-determined rotational speed. The rotational motion of the rollers 14 is transformed into a linear motion 26 in respect to the surface of the wafer 24. This is shown in FIG. 1B.
In the linear polisher 10, a polishing pad 30 is adhesively joined to the continuous belt 12 on its outer surface that faces the wafer 24. A polishing assembly 40 is thus formed by the continuous belt 12 and the polishing pad 30 glued thereto. As shown in FIG. 1A, a plurality of polishing pads 30 are utilized which are frequently supplied in rectangular-shaped pieces with a pressure sensitive layer coated on the back side.
The wafer platform 18 and the wafer 24 forms an assembly of a wafer carrier 28. The wafer 24 is normally held in position by a mechanical retainer, commonly known as a retaining ring 16, as shown in FIG. 1B. The major function of the retaining ring 16 is to fix the wafer in position in the wafer carrier 28 during the linear polishing process and thus preventing the wafer from moving horizontally as wafer 24 contacts the polishing pad 30. The wafer carrier 28 is normally operated in a rotational mode such that a more uniform polishing on wafer 24 can be achieved. To further improve the uniformity of linear polishing, a support housing 32 is utilized to provide support to support platen 22 during a polishing process. The support platen 22 provides a supporting platform for the underside of the continuous belt 12 to ensure that the polishing pad 30 makes sufficient contact with the surface of wafer 24 in order to achieve more uniform removal in the surface layer. Typically, the wafer carrier 28 is pressed downwardly against the continuous belt 12 and the polishing pad 30 at a predetermined force such that a suitable polishing rate on the surface of wafer 24 can be obtained. A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting forces on the support housing 32, the wafer carrier 28, and the linear speed 26 of the polishing pad 30. A slurry dispenser 20 is further utilized to dispense a slurry solution 34.
In the conventional linear polisher 10, the polishing pads 30 are joined to the continuous belt 12 by adhesive means such as by a pressure sensitive. In a typical linear polisher, since the continuous belt 12 may have a length of about 240 cm, while the polishing pads 30 cannot be supplied in the form of a continuous manner, many pieces of the polishing pads 30 must be used. In other words, seam lines between adjacent polishing pads 30 must be formed when joined to the continuous belt 12. For instance, when the polishing pads are supplied in length of only about 30xcx9c40 cm, between five and seven pieces of the polishing pads must be utilized.
The linear chemical mechanical polishing method provides the advantages of a high belt speed, a low compression force on the sample and the flexibility of using either a hard pad or a soft pad. However, the seam lines discussed above frequently cause problems for a semiconductor wafer that is being polished in the linear polishing process. For instance, as shown in FIG. 2, the polishing pads 30 are adhesively bonded to the linear belt 12 which is made of stainless steel and stretched in a highly tensioned state, i.e. stretched over the rollers 14. It is impossible to have a tight seam or joint between adjacent polishing pads 30. Instead, a gap xe2x80x9c1xe2x80x9d is frequently formed between the polishing pads. The gap xe2x80x9c1xe2x80x9d may have a magnitude between about 1 mm and about 5 mm.
Furthermore, since the polishing pad 30 is normally formed of a hard pad material, i.e. having a Durometer A hardness of at least 60, the edge 40 of the polishing pad 30 may have a rough edge due to the cutting process in which the pads are trimmed to size. The rough edge exists regardless of whether the pad is cut at 90xc2x0 angle or at 45xc2x0 angle to the length of the pad.
During the linear polishing process, the wafer platform 18 presses wafer 24 onto the polishing pad 30 under a predetermined compressing force. Both the flexibility of the polishing pad 30 and the flexibility of the linear belt 12 allows the top surface of the wafer 24 to be pushed below the top surface of the polishing pad 30. As a result, as the rollers 14 are turned to move linearly the belt 12 and the polishing pad 30 mounted thereon, a front edge (or leading edge) 40 of the polishing pad 30 constantly collides with the front edge of the wafer 24. Such collision or impact on the wafer edge by the leading edge 40 of the polishing pad 30 causes a delamination or peling of the coating layers on the wafer surface. For instance, this type of polishing defect has been known to occur in wafers that have low-k oxide film layers deposited on top. The peeling or delamination of the low-k oxide films from the wafer surface at the wafer edge severely affects the yield of the fabrication process. The defect must therefore be minimized or prevented altogether.
It is therefore an object of the present invention to provide a composite polishing pad for use in a linear polishing process that does not have the drawbacks or shortcomings of the conventional polishing pads.
It is another object of the present invention to provide a composite polishing pad for use in a linear polishing process that does not cause film peeling or delamination from a wafer surface that is being polished.
It is a further object of the present invention to provide a composite polishing pad for use in a linear polishing process in which the composite polishing pad is constructed of a hard pad material with a soft pad laminated thereon.
It is another further object of the present invention to provide a dual-hardness polishing pad for use in a linear polishing process in which a pad body is formed of a material that has a first hardness, and a pad cover is formed of a material that has a second hardness which is at least 20% lower than the first hardness.
It is still another object of the present invention to provide a dual-hardness polishing pad for use in a linear chemical mechanical polishing process in which a soft pad material is used to cover a leading edge of a polishing pad made of a hard pad material.
It is yet another object of the present invention to provide a dual-hardness polishing pad for use in a linear chemical mechanical polishing process in which a pad body is formed of a first material having a Durometer A hardness of at least 60, which is coated on a leading edge by a pad material of a second hardness that is at least 20% lower than the first hardness.
It is still another further object of the present invention to provide a method for forming a dual-hardness polishing pad for use in a linear polisher by first forming a body portion of the pad in a hard pad material and then covering a leading edge of the body portion with a soft pad material.
It is yet another further object of the present invention to provide a method for forming a dual-hardness polishing pad for use in a linear polisher by first providing a body portion made of a material that has a Durometer A hardness of at least 60, and then coating a leading edge of the body portion with a second material that has a Durometer A hardness of less than 50.
In accordance with the present invention, a composite, dual-hardness polishing pad for use in a linear chemical mechanical polishing process and a method for forming the pad are provided.
In a preferred embodiment, a composite polishing pad for linear polishing is provided which includes a pad body that has a leading edge and a trailing edge for mounting to a linear belt immediately adjacent to a second polishing pad. The pad body is fabricated of a material that has a first hardness, the leading edge contacts an object being polished on the composite polishing pad before the trailing edge when the linear belt turns in a linear polishing process, and a buffer pad adhesively joined to the leading edge of the pad body for contacting the object that is being polished, the buffer pad is fabricated of a material that has a second hardness which is at least 20% lower than the first hardness such that impact on the object is minimized during a linear polishing process.
In the composite polishing pad for linear polishing, the pad body is mounted to the linear belt immediately adjacent to the second polishing pad with a space of at least 1 mm therein-between. The pad body may be mounted to the linear belt by adhesive means, and may be fabricated of a material that has a Durometer A hardness not lower than 60. The buffer pad may be fabricated of a material that has a Durometer A hardness not higher than 50. The buffer pad adhesively joins and covers both a horizontal top surface and a vertical side surface of the leading edge of the pad body.
The present invention is further directed to a dual-hardness polishing pad for use in linear chemical mechanical polishing which includes a body portion formed of a first material that has a first hardness, the body portion has a leading edge and a trailing edge and when mounted to a linear belt and rotated during a linear polishing process, the leading edge contacts an object being polished first before the trailing edge; and a cover portion formed of a second material that has a second hardness which is at least 20% lower than the first hardness, the cover portion substantially covers the leading edge of the body portion such that impact on the object being polished can be minimized during a linear polishing process.
In the dual-hardness polishing pad for use in linear chemical mechanical polishing, the body portion and the cover portion of the polishing pad are formed of a polymeric material. The first material may have a Durometer A hardness of not lower than 60, while the second material may have a Durometer A hardness of not higher than 50. The dual-hardness polishing pad is mounted to a linear belt juxtaposed to a second dual-hardness polishing pad with a spacing of at least 1 mm therein-between. The cover portion substantially covers the leading edge of the body portion on both a horizontal top surface and on a vertical side surface.
The present invention is still further directed to a method for forming a dual-hardness polishing pad for use in a linear polisher which includes the operating steps of first providing a body portion that has a leading edge and a trailing edge, the body portion being formed of a material that has a first hardness; then providing a cover portion that has a contour for intimately joining the leading edge of the body portion, the cover portion being formed of a material that has a second hardness that is at least 20% lower than the first hardness; and joining the cover portion to the body portion forming the dual-hardness polishing pad.
The method for forming a dual-hardness polishing pad for use in a linear polisher may further include the step of joining the cover portion to the body portion by adhesive means, or the step of forming the body portion with a material that has a Durometer A hardness of at least 60, or the step of forming the cover portion with a material that has a Durometer A hardness of not higher than 50. The method may further include the step of joining the cover portion to the body portion by substantially covering both a horizontal top surface and a vertical side surface of the body portion.